Tube pedal guys - so what about Cascode for a very high overall level of gain?

[earthtonesaudio]

I think cascodes and active loads/constant current sources are very different.  All that's implied by an active load is a dynamic/AC impedance much higher than the DC resistance.  With a cascode stage you actually have a separate voltage gain stage.  The lines do get a bit blurry around mu-amps and SRPP stages though.
But there's no reason not to combine them... SRPP loaded cascode: 3 tubes stacked "vertically," high gain and low output impedance.  But by that point you're getting into op-amp territory...

[brett]

Hi
thanks for pointing out the differences between cascodes and active loads.  Note, however, that some circuits seem to have active loads that also provide a gain stage. The Jordan Bosstone has a PNP device sitting on the collector of an NPN device, providing a very active load, low output impedance, and a gain of 2x (or thereabouts).  Although 2x doesn't seem much, BJTs in the early 1970s typically had hFE around 200.  Doubling that and adding some clipping diodes made for a very respectable distortion pedal in its day.
cheers

[earthtonesaudio]

Not claiming to be a Bosstone expert by any stretch, but this: http://www.geofex.com/FX_images/bosstone.gif looks to me that the PNP provides no voltage gain itself, but is bootstrapping the NPN's collector resistor to raise the gain of the NPN stage.  The lack of bootstrap capacitor makes it also look kinda like a CCS, so maybe I'm way off.

But it certainly is a good example of there being many valid ways of wringing more gain from the poor little discrete devices, be they solid state or tube!  Ah, if only there were P-channel tubes... darn you physics!

[Paul Marossy]

The Seymour Duncan Convertible had a Cascode Module available (and I in fact have one). In the owner's manual, it claims that this module has "a bit more gain and brighter, crisp tone. Lots of detail".

I've never heard the supposition that it's a way to get more gain, but I suppose that it could also be used for such purposes. My understanding is that it's supposed to give higher fidelity than other methods of tube amplification.

[brett]

Hi
Wikipedia describes a cascode as a transconductance amplifier followed by a current buffer.  That seems to "fit" our discussions.  This

may have one or more of the following advantages: higher input-output isolation, higher input impedance, higher output impedance, higher gain or higher bandwidth

Thinking about what is happening here, it is clear that (unlike the Bosstone), the lower device isn't making much voltage gain (and tyhat's why cascodes were popular for avoiding Miller effect).  In fact, the lower device is producing quite low gain, but if the webpages I've looked at are correct, the total gain is the *product* of the transconductunce (and ro) of the two FETs/tubes.  The formula for the voltage gain is Av = − (gm1.ro1 + 1).gm2.ro2 (where, for a J201, gm is approx 5 mA/V.)  These calculations assume that the current flow into the grid or gate are negligible.  A different formula will apply for BJTs.
cheers

[Sir H C]

The reason a cascode will not buy much here is that you still have a resistive load (and a pretty small one on a voltage starved circuit) so that will limit the gain of the stage.  If you put an active load above the cascode then you can get some super high gain, but controlling the bias point is tricky.  So cascoding might buy something small with a tube with a poor output impedance but not huge unless you fix the load.

[DougH]

From my experience with the Firefly, I think a cascode is more trouble than it's worth. It will give you more gain than you will ever need or could use, at the cost of introducing a lot more noise. When I had my Firefly I never turned the cascode volume above 50% (and that was a log pot so that was actually 10%). Above that, it was pretty useless. If I had to do it over, I would have added a simple CC gain stage and called it a day.

[Gus]

Some people even change to 12at7s and 12ay7s from 12ax7s to lower gain in amps.

Cascode reduces the miller cap affect on the stage.

http://www.runoffgroove.com/fetzervalve.html